Polysilicon resistor formation in silicon-on-insulator replacement metal gate finFET processes

ABSTRACT

A method of forming a polysilicon resistor in replacement metal gate (RMG) processing of finFET devices includes forming a plurality of semiconductor fins over a buried oxide layer of a silicon-on-insulator substrate; forming a trench in the buried oxide layer; forming a polysilicon layer over the semiconductor fins and in the trench, the polysilicon layer having a depression corresponding to a location of the trench; forming an insulating layer over the polysilicon layer, and performing a planarizing operation to remove the insulating layer except for a portion of the insulating layer formed in the depression, thereby defining a protective island; patterning the polysilicon layer to define both a dummy gate structure over the fins and the polysilicon resistor; and etching the polysilicon layer to remove the dummy gate structure, wherein the protective island prevents the polysilicon resistor from being removed.

BACKGROUND

The present invention relates generally to semiconductor devicemanufacturing and, more particularly, to polysilicon resistor formationin silicon-on-insulator (SOI), replacement metal gate (RMG) finFETprocesses.

Field effect transistors (FETs) are widely used in the electronicsindustry for switching, amplification, filtering, and other tasksrelated to both analog and digital electrical signals. Most common amongthese are metal-oxide-semiconductor field-effect transistors (MOSFET orMOS), in which a gate structure is energized to create an electric fieldin an underlying channel region of a semiconductor body, by whichelectrons are allowed to travel through the channel between a sourceregion and a drain region of the semiconductor body. Complementary MOS(CMOS) devices have become widely used in the semiconductor industry,wherein both n-type and p-type (NMOS and PMOS) transistors are used tofabricate logic and other circuitry.

The source and drain regions of an FET are typically formed by addingdopants to targeted regions of a semiconductor body on either side ofthe channel. A gate structure is formed above the channel, whichincludes a gate dielectric located over the channel and a gate conductorabove the gate dielectric. The gate dielectric is an insulator material,which prevents large leakage currents from flowing into the channel whena voltage is applied to the gate conductor, while allowing the appliedgate voltage to set up a transverse electric field in the channel regionin a controllable manner. Conventional MOS transistors typically includea gate dielectric formed by depositing or by growing silicon dioxide(SiO₂) or silicon oxynitride (SiON) over a silicon wafer surface, withdoped polysilicon formed over the SiO₂ to act as the gate conductor.

The escalating demands for high density and performance associated withultra large scale integrated (ULSI) circuit devices have requiredcertain design features, such as shrinking gate lengths, highreliability and increased manufacturing throughput. The continuedreduction of design features has challenged the limitations ofconventional fabrication techniques.

For example, when the gate length of conventional planar metal oxidesemiconductor field effect transistors (MOSFETs) is scaled below 100 nm,problems associated with short channel effects (e.g., excessive leakagebetween the source and drain regions) become increasingly difficult toovercome. In addition, mobility degradation and a number of processissues also make it difficult to scale conventional MOSFETs to includeincreasingly smaller device features. New device structures aretherefore being explored to improve FET performance and allow furtherdevice scaling.

Double-gate MOSFETs represent one type of structure that has beenconsidered as a candidate for succeeding existing planar MOSFETs. Indouble-gate MOSFETs, two gates may be used to control short channeleffects. A finFET is a double-gate structure that exhibits good shortchannel behavior, and includes a channel formed in a vertical fin. ThefinFET structure may be fabricated using layout and process techniquessimilar to those used for conventional planar MOSFETs.

SUMMARY

In one aspect, a method of forming a polysilicon resistor in replacementmetal gate (RMG) processing of finFET devices includes forming aplurality of semiconductor fins over a buried oxide layer of asilicon-on-insulator substrate; forming a trench in the buried oxidelayer; forming a polysilicon layer over the semiconductor fins and inthe trench, the polysilicon layer having a depression corresponding to alocation of the trench; forming an insulating layer over the polysiliconlayer, and performing a planarizing operation to remove the insulatinglayer except for a portion of the insulating layer formed in thedepression, thereby defining a protective island; patterning thepolysilicon layer to define both a dummy gate structure over the finsand the polysilicon resistor; and etching the polysilicon layer toremove the dummy gate structure, wherein the protective island preventsthe polysilicon resistor from being removed.

In another aspect, a method of forming a polysilicon resistor inreplacement metal gate (RMG) processing of finFET devices includesforming a plurality of semiconductor fins over a buried oxide layer of asilicon-on-insulator substrate; forming a trench in the buried oxidelayer; forming conformal oxide layer on the semiconductor fins, theburied oxide layer, and the trench; forming a polysilicon layer over thesemiconductor fins and in the trench, the polysilicon layer having adepression corresponding to a location of the trench; forming an oxidelayer over the polysilicon layer; planarizing the oxide layer and thepolysilicon layer so as to remove the oxide layer, except for a portionof the oxide layer formed in the depression, thereby defining aprotective oxide island directly over a portion of the polysilicon layercorresponding to a location of the polysilicon resistor; forming anitride hardmask over the polysilicon layer and the protective oxideisland; patterning the hardmask and etching the polysilicon layer todefine both a dummy gate structure over the fins and the polysiliconresistor; doping the polysilicon resistor; depositing a high densityplasma (HDP) oxide layer and planarizing the HDP layer to expose thepatterned hardmask layer; removing the patterned hardmask layer andetching the exposed polysilicon layer to remove the dummy gatestructure, wherein the protective oxide island prevents the polysiliconresistor from being removed; and forming one or more replacement metalgate stack layers over the fins.

In another aspect, a semiconductor device includes asilicon-on-insulator substrate including a bulk layer, a buried oxidelayer on the bulk layer, and a plurality of semiconductor fins formed onthe buried oxide layer; a polysilicon resistor disposed in a trenchformed within the buried oxide layer; and one or more replacement metalgate layers formed over the semiconductor fins.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the several Figures:

FIGS. 1 through 14 are a series of cross sectional views of a method offorming resistor devices, in accordance with an exemplary embodiment, inwhich:

FIG. 1 illustrates a starting SOI substrate;

FIG. 2 illustrates the formation of one or more semiconductor fins inthe SOI layer of the SOI substrate;

FIG. 3 illustrates a photoresist pattern formed over the structure ofFIG. 2;

FIG. 4 illustrates an etch process to form a recess within the BOX layerof the SOI substrate;

FIG. 5 illustrates the formation of a conformal oxide layer followed bya polysilicon layer deposition;

FIG. 6 illustrates a blanket oxide deposition over the polysiliconlayer;

FIG. 7 illustrates a planarizing operation to remove most of the oxidematerial, leaving an oxide island directly over the recess in the BOXlayer;

FIG. 8 illustrates a hardmask layer formed over the structure of FIG. 7;

FIG. 9 illustrates patterning of the hardmask layer and etching todefine both a dummy gate region and a polysilicon resistor;

FIG. 10 illustrates doping of the polysilicon resistor;

FIG. 11 illustrates forming a high density plasma (HDP) oxide layerfollowed by planarizing to expose remaining portions of the hardmasklayer;

FIG. 12 illustrates removal of the hardmask layer and the dummypolysilicon gate material;

FIG. 13 illustrates the formation of high-k, workfunction and gate metallayers on the structure of FIG. 12; and

FIG. 14 illustrates planarizing the gate stack layers.

DETAILED DESCRIPTION

Polysilicon resistors have been widely used in conventional integratedcircuit design, including for resistor capacitor (RC) oscillators,current limitation resistance, electrostatic discharge (ESD) protection,radio frequency (RF) post drivers, on-chip termination, impedancematching, etc. In traditional or gate first fabrication techniques forfinFET devices that include a polysilicon resistor or other such passivestructures, the polysilicon formation is used for both the gate stack aswell as for the resistor.

On the other hand, with replacement metal gate (RMG) or gate lasttechnology for finFET devices, a material such as polysilicon is used todefine a dummy gate structure over the semiconductor fins prior tosource/drain definition, doping, epitaxial fin merging, etc. Thereafter,the dummy gate material is selectively removed from the structurefollowed by formation of the final device gate stack materials, such asone or more high-k dielectric layers, metal workfunction layers andmetal gate conductor layers. Thus, polysilicon resistors are not easilyintegrated into RMG finFET processing due to the subsequent removaloperation of dummy polysilicon gate material.

Presently, metal resistors are commonly used in RMG finFET processing inlieu of polysilicon resistors. However, the use of metal for theresistor is not as advantageous as polysilicon, since in order to have alarger range of resistance values it is generally necessary to have awider variety of sizes for the metal resistor given a fairly constantresistivity value. In contrast, polysilicon resistors offer flexibilityin terms of resistance variation for a given size, using appropriateadjustments in doping of the resistor, to achieve resistance values ofabout 200-1000 ohms per square (Ω/□) for example. Therefore, it would bedesirable to be able to integrate polysilicon resistor formation intoSOI RMG finFET processing.

Accordingly, disclosed herein is a method of forming polysiliconresistors in SOI, RMG finFET processes. By forming a recess in theburied oxide (BOX) layer of the SOI substrate corresponding to thedesired location of a polysilicon resistor, a subsequent polysiliconlayer deposition (for both dummy gate and resistor use) will assume asimilar topography such that a protective oxide layer may be formed in acorresponding recess above the resistor polysilicon. This protectiveoxide layer remains as an “oxide island” after a planarizing operation,and will protect the polysilicon resistor during removal of the dummygate polysilicon material over the fin structures.

Referring initially to FIG. 1, there is shown a cross sectional view ofa starting silicon-on-insulator (SOI) substrate 100 suitable for use inaccordance with exemplary embodiments. The SOI substrate 100 includes abulk semiconductor layer 102 (e.g., silicon), a buried insulator oroxide layer (BOX) 104 formed on the bulk semiconductor layer 102, and asemiconductor (e.g., silicon) layer 106 formed on the BOX layer 104. Asshown in FIG. 2, one or more semiconductor fins 108 are formed in theSOI layer 106 using any technique suitable in the art, includingphotoresist/hardmask patterning and etching, sidewall image transfer(SIT), and the like.

In FIG. 3, a photoresist layer 110 is formed over the structure of FIG.2, and an opening or trench 112 is patterned into the photoresist layer110 corresponding to a desired location of a polysilicon resistor to beformed in later processing operations. The trench 112 is thentransferred into a portion of the BOX layer 104, as shown in FIG. 4.Once the trench 112 is formed in the BOX layer 104, the photoresistlayer may then be removed.

Referring now to FIG. 5, a thin oxide layer (e.g., SiO₂) 114 isconformally deposited over exposed surfaces, including the top surfaceof the BOX layer 104, including side and bottom surfaces of the trench112, and top and side surfaces of the semiconductor fins 108. The oxidelayer 114 may be formed by atomic layer deposition (ALD), for example,to an exemplary thickness of about 2-4 nanometers (nm). The oxidedeposition is then followed by deposition of a polysilicon layer 116,which serves as both a dummy gate material over the fins 108, as well asthe subsequently defined polysilicon resistor. The oxide layer 114 hasan exemplary thickness of about 30-100 nm, and it will be noted that theoxide layer 114 has a topography that generally mirrors that of theunderling surfaces. In particular, it will also be seen from FIG. 5 thatthere is a depression 118 in the oxide layer 114 corresponding to thetrench 112 defined in the BOX layer 104.

As then shown in FIG. 6, an oxide layer 120 (e.g., SiO₂) is blanketdeposited over the topographic polysilicon layer 116 completely fillingthe depression 118. A chemical mechanical planarizing/polishing (CMP)operation is then used to remove most of the oxide layer 120 andplanarize the dummy gate portion of the oxide layer 116. This isillustrated in FIG. 7, which also shows that a protective “oxide island”122 remains over the portion of the polysilicon layer 116 formed in therecess of the BOX layer 104. The protective oxide island 122 ultimatelyprotects the polysilicon resistor at a point in processing when removingdummy gate polysilicon material, as will be described in further detail.

Referring to FIG. 8, a hardmask layer 124 is formed over the structureof FIG. 7, including planarized top surfaces of the polysilicon layer116 and the protective oxide island 122. The hardmask layer 124 mayinclude any suitable material, such as nitride layer for example, thathas an etch selectivity with respect to oxide and polysilicon materials.As shown in FIG. 9, the hardmask layer 124 is patterned, followed byetching to remove portions of the polysilicon layer 116 not used for thedummy gate structure or for the polysilicon resistor 126. It will benoted that the protective oxide island 122 remains over the newlydefined polysilicon resistor 126.

After the dummy gate and resistor definition in FIG. 9, FIG. 10illustrates the formation and patterning of a resist layer 128 to exposethe region including the polysilicon resistor 126. An ion implantation(e.g., boron), indicated by the arrows in FIG. 10, is used to dope theresistor (now designated by 126′) to a desired conductivity. The desiredconductivity of the doped resistor 126′ depends on the concentration ofthe dopant atoms and implantation energy, among other aspects. Ingeneral, the implant conditions are selected so as place a majority ofthe dopant concentration roughly at a mid-region of the resistor 126′.

With the resistor 126′ now doped to have the desired resistance value,the resist layer may then be removed. As then shown in FIG. 11, a highdensity plasma (HDP) oxide layer 130 is formed over the structure, andthen planarized to expose remaining portions of the hardmask layer 124.The hardmask layer 124 is then selectively removed to expose the dummygate polysilicon material 116 over the fins 108. Notably, the hardmaskremoval leaves the protective oxide island 126 substantially intact. InFIG. 12, an etch process to remove the dummy gate polysilicon materialis performed, leaving the resistor 126′ in place. The thin conformaloxide layer 114 may optionally be removed after the polysilicon removal,in preparation for the replacement gate stack formation. Alternatively,it may remain in place to act as an interfacial layer for a high-k gatedielectric layer.

As then shown in FIG. 13, RMG processing as known in the art maycontinue, including the formation of high-k, workfunction and gate metallayers. For ease of illustration, the gate stack layers are generallyindicated by 132 in FIG. 13, and it will be understood that the layers132 may include several materials. Finally, as shown in FIG. 14, thegate stack material layers 132 are planarized to define a finFET areaand a doped polysilicon resistor 126′. From this point, processing maycontinue as known in the art, including forming FET and resistor contactstructures, and upper level wiring.

As will thus be appreciated, the topographic deposition of polysiliconmaterial followed by oxide deposition allows for an aligned, protectiveoxide cap to cover resistor polysilicon material in a replacement gateprocess for SOI finFET devices such that dummy gate material removaldoes not affect the integrity of the resistor.

While the invention has been described with reference to a preferredembodiment or embodiments, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor elements thereof without departing from the scope of the invention.In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

What is claimed is:
 1. A method of forming a polysilicon resistor inreplacement metal gate (RMG) processing of finFET devices, the methodcomprising: forming a plurality of semiconductor fins over a buriedoxide layer of a silicon-on-insulator substrate; forming a trench in theburied oxide layer; forming a polysilicon layer over the semiconductorfins and in the trench, the polysilicon layer having a depressioncorresponding to a location of the trench; forming an insulating layerover the polysilicon layer, and performing a planarizing operation toremove the insulating layer except for a portion of the insulating layerformed in the depression, thereby defining a protective island;patterning the polysilicon layer to define both a dummy gate structureover the fins and the polysilicon resistor; and etching the polysiliconlayer to remove the dummy gate structure, wherein the protective islandprevents the polysilicon resistor from being removed.
 2. The method ofclaim 1, further comprising doping the polysilicon resistor.
 3. Themethod of claim 1, further comprising, following removing the dummy gatestructure, forming one or more replacement metal gate stack layers overthe fins.
 4. The method of claim 1, further comprising forming conformaloxide layer over the semiconductor fins, the buried oxide layer, and thetrench prior to forming the polysilicon layer.
 5. The method of claim 4,wherein the conformal oxide layer has a thickness of about 2-4nanometers (nm).
 6. The method of claim 5, wherein the polysilicon layerhas a thickness of about 30-100 nm.
 7. A method of forming a polysiliconresistor in replacement metal gate (RMG) processing of finFET devices,the method comprising: forming a plurality of semiconductor fins over aburied oxide layer of a silicon-on-insulator substrate; forming a trenchin the buried oxide layer; forming conformal oxide layer on thesemiconductor fins, the buried oxide layer, and the trench; forming apolysilicon layer over the semiconductor fins and in the trench, thepolysilicon layer having a depression corresponding to a location of thetrench; forming an oxide layer over the polysilicon layer; planarizingthe oxide layer and the polysilicon layer so as to remove the oxidelayer, except for a portion of the oxide layer formed in the depression,thereby defining a protective oxide island directly over a portion ofthe polysilicon layer corresponding to a location of the polysiliconresistor; forming a nitride hardmask over the polysilicon layer and theprotective oxide island; patterning the hardmask and etching thepolysilicon layer to define both a dummy gate structure over the finsand the polysilicon resistor; doping the polysilicon resistor;depositing a high density plasma (HDP) oxide layer and planarizing theHDP layer to expose the patterned hardmask layer; removing the patternedhardmask layer and etching the exposed polysilicon layer to remove thedummy gate structure, wherein the protective oxide island prevents thepolysilicon resistor from being removed; and forming one or morereplacement metal gate stack layers over the fins.
 8. The method ofclaim 7, wherein the conformal oxide layer has a thickness of about 2-4nanometers (nm).
 9. The method of claim 8, wherein the polysilicon layerhas a thickness of about 30-100 nm.
 10. The method of claim 7, whereindoping the polysilicon resistor further comprises forming a patternedphotoresist layer to protect the semiconductor fins and performing adopant implant to implant dopant atoms in the polysilicon resistor. 11.The method of claim 10, wherein the dopant atoms comprise boron.